In many disease states, doctors want to deliver a therapeutic agent to a target area and have the agent remain in the target area to treat the tissue for an extended period of time. If the therapeutic agent has the consistency of liquid, the body quickly and efficiently carries it away from the target area. As a result, the duration of time that the agent has to treat the target area is short.
In order to reduce the body's ability to carry the therapeutic agent away from the target area others have increased the viscosity of the therapeutic agent such that it has the consistency of gel. Using gels is effective because the diffusion rates of gels are slower than the diffusion rates of liquids. Thus the gel elutes the therapeutic agent over a longer time course treating the target area for a greater period of time. There are many types of gels that can incorporate therapeutic agents. Each type of gel changes viscosity in response to different environments. Those environments include pH, temperature, catalyst, chemical reaction, solvent, or reaction with compounds in the body.
Catheters are traditionally used to deliver therapeutic agents to remote target areas in the body. Generally catheters traverse the vascular pathways of the body until the tip of the catheter reaches the target area. Catheters are used in the cardiovascular, gastric, general, urological, neurological, and oncological fields of medicine. The target area can be a blood vessel, an organ, or a tumor. Catheters have been used to deliver acute therapeutic agents such as analgesic, antibacterial, anti-restenotic, anticancer, anti-inflammatory agents and hormones, and bulking agents such as collagen and stainless steel micro-embolic coils. However, due to the small size and long length of the catheters they cannot deliver viscous materials such as gels to remote sites in the body.
One way to circumvent the problem of delivering viscous materials is to use a special class of gels, known as reverse-thermal gelation gels (also known by the trademarked name PLURONIC® gels or TETRONIC® gels, available from BASF and other suppliers). These gels are characterized by the property of being liquid below a critical solution temperature and becoming viscous, or gel-like, above the critical solution temperature. This is in contrast with normal matter that is solid below a critical temperature, for example, the freezing temperature and liquid above that critical temperature, exhibiting decreased viscosity as the temperature of the matter increases. The critical solution temperature of these gels can be tailored by their chemistry such that they are liquid at room temperature or below (0–23° C.) and gel at body temperature (37° C.).
There have been many studies of the biological behavior of sustained release of substances from PLURONIC® gels in animals. This gel has typically been delivered to shallow tissue in animals through a syringe. It is possible to deliver it in this manner because the delivery path is short enough that the liquid doesn't gel before it is delivered through the syringe tip because the animal's body heat does not have sufficient time to heat it up.
Gels, however, cannot be delivered to remote sites in the body such as the heart or the brain through a catheter because the gels are too viscous to be injected.